Boron blue is a glaze fault involving the crystallization of calcium, boron and silicate compounds. It can be solved using ceramic chemistry.
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Boron blue is the bluish haze or clouding in a transparent boron glaze that results from the crystallization of calcium borate in the glass matrix during cooling. While the effect might be considered decorative in some cases, this is generally considered a glaze fault. It is a common problem in high boron glazes, the higher the CaO the worse it is. It is about the chemistry of the glaze (even though some might like to blame the frit).
If possible, try cooling faster (of course there is a risk for dunting with this approach). That gives the crystals less time to grow.
The most effective approach is to reduce the CaO. As long as it is not excessive the crystals should not form. Even if the glaze has plenty of boron.
If the glaze is melting well it will likely tolerate the addition of more Al2O3. Increasing it without changing anything else impedes the growth of crystals (by stiffening the melt thereby denying the molecules the mobility they need to form the preferred crystalline arrangement). There are added benefits to this also (e.g. better hardness, lower expansion).
These changes are done using glaze chemistry. If you enter a recipe into your account at Insight-live.com it can show you the amount of each oxide (e.g. CaO, Al2O3, SiO2) and you can click materials to see what oxides they contribute. Then duplicate the recipe and viewing it side-by-side with the original. Adjust ingredients and note the impact on the chemistry. It may be as simple as changing some or all of an existing frit in the recipe for another frit (or mix of frits) having higher Al2O3 and a mix of fluxes having a lower percentage of CaO. A common way to add Al2O3 is to increase the kaolin and reduce the silica (to re-match the SiO2).
The most non-technical way to adjust the recipe is to simply blindly try other frits. While this might reduce the boron blue, the different chemistry they bring can affect other properties (like color response, thermal expansion, melting temperature, hardness and durability, surface character, matteness or gloss). Since the root problem is with two oxides the best approach is to focus on them.
Boron blue can also be used as a decorative effect, especially on low-temperature ware. To encourage it just do the opposite of everything mentioned above.
This high boron cone 04 glaze is generating calcium-borate crystals during cool down (called boron-blue). This is a common problem and a reason to control the boron levels in transparent glazes; use just enough to melt it well. If more melt fluidity is needed, decrease the percentage of CaO. There is a positive: For opaque glazes, this effect can actually enable the use of less opacifier.
Two clear glazes fired in the same slow-cool kiln on the same body with the same thickness. Why is one suffering boron blue (1916Q) and the other is not? Chemistry and material sourcing. Boron blue crystals grow best when there is plenty of boron (and other power fluxes), alumina is low, adequate silica is available and cooling is slow enough to give them time to grow. In the glaze on the left B2O3 is higher, crystal-fighting Al2O3 and MgO levels are a lot lower, KNaO fluxing is significantly higher, it has more SiO2 and the cooling is slow. In addition, it is sourcing B2O3 from a frit making the boron even more available for crystal formation (the glaze on the right is G2931F, it sources its boron from Ulexite).
It is made from 85% Ferro Frit 3134, 7.5% kaolin and 7.5% silica. While not obvious from the recipe, one look at the chemistry of this (as displayed when you enter a recipe into your account at insight-live.com) will show very low Al2O3. Frit 3134 has almost no Al2O3, yet it is an essential component of functional glazes (for durability, resistance to crystallization, stability during firing). The kaolin is the only contributor of Al2O3 and there is only a little. A simple fix would be to use Ferro Frit 3124 instead, remove the silica and increase the kaolin to 15.
This happens. They are glossy, but lack thickness and body. They are also prone to boron blue clouding (micro crystallization that occurs because low alumina melts crystallize much more readily on cooling). Another problem is lack of resistance to wear and to leaching (sufficient Al2O3 in the chemistry is essential to producing a strong and durable glass). This is a good example of the need to see a glaze not just as a recipe but as a chemical formula of oxides. The latter view enables us to compare it with other common recipes and the very low Al2O3 is immediately evident. Another problem: Low clay content (this has only 7.5% kaolin) creates a slurry that is difficult to use and quickly settles hard in the bucket.
To get the needed chemistry to avoid boron blue clouding (calcium borate crystals). The one on the right clouds, the other does not. Why? Differences in the chemistry (as seen in my account at insight-live.com). G2931K, on the left, has greater Al2O3 (which impedes the growth of crystals), lower CaO (starves their growth) and more boron (for better melting). There is actually no practical way to adjust the recipe on the right (by supplying MgO with talc and fiddling with frit percentages) to achieve this. Frit 3124 lacks Na2O and B2O3. 3134 has excessive CaO and almost zero Al2O3. Talc does not melt well enough. But Frit 3249 supplies the needed MgO and has lots of B2O3 and low CaO. And Frit 3110 has low CaO and supplies the needed Na2O.
Example of variegation by thickness-induced boron blue
A pottery glaze so melt-fluid it can eat through a firebrick. The fix struck boron-blue gold!
Glaze opacity refers to the degree to which it is opaque. Opacifiers are powders added to transparent ceramic glazes to make them opaque.
Every glossy ceramic glaze is actually a base transparent with added opacifiers and colorants. So understand how to make a good transparent, then build other glazes on it.
Opacity of ceramics glazes is normally achieved by adding an opacifier like tin oxide or zircon. However, there are chemical profiles that can turn transparent glazes milky and make it cheaper to opacify them.
Ceramic glazes form crystals on cooling if the chemistry is right and the rate of cool is slow enough to permit molecular movement to the preferred orientation.
Glaze chemistry is the study of how the oxide chemistry of glazes relate to the way they fire. It accounts for color, surface, hardness, texture, melting temperature, thermal expansion, etc.
G1216M - Cone 6 Ultraclear Glaze for Porcelains
Substitute for low expansion cone 6 G1215U, this sources MgO from talc instead of a frit
Gerstley Borate was a natural source of boron for ceramic glazes. It was plastic and melted clear at 1750F. Now we need to replace it. How?
|Al2O3 - Aluminum Oxide, Alumina
|By Tony Hansen
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